In manufacture of a semiconductor including LSI, super LSI and the like or in manufacture of a liquid crystal display or the like, a pattern is made by irradiating light to a semiconductor wafer or an original plate for liquid crystal, but if a dust adheres to a photomask or a reticle (hereinafter simply referred to as a photomask in short) used at this time, the dust absorbs light or refracts the light, which causes deformation of a transferred pattern, rough edges or black stains on a base and leads to a problem that dimensions, quality, appearance and the like are damaged.
Thus, these works are usually performed in a clean room, but it is still difficult to keep the photomask clean all the time. Therefore, a pellicle is attached to the surface of the photomask as a dustproof and then, exposure is carried out. In this case, foreign substances do not directly adhere to the surface of the photomask but to the pellicle, and by setting a focus on a pattern of the photomask in lithography, the foreign substances on the pellicle no longer become a concern to the transfer.
A pellicle has a transparent pellicle film attached on an upper end face of a pellicle frame. A material of the pellicle film includes cellulose nitrate, cellulose acetate, fluorine contained resin or the like, which transmits light well, and a material of the pellicle frame includes aluminum, stainless, polyethylene and the like. Moreover, a pressure-sensitive adhesive layer made of polybutene resin, polyvinyl acetate resin, acrylic resin and the like to be attached to the photomask and a separation layer (separator) intended to protect the pressure-sensitive adhesive layer are disposed at a lower end of the pellicle frame in general.
Also, in a state in which the pellicle is attached to the photomask, with the purpose of eliminating a pressure difference between a space contained inside the pellicle and the outside, one or more ventilation hole for adjusting pressure is bored through pellicle frame bar(s), and a filter member is installed in order to prevent foreign substances from the atmosphere from moving in and through the ventilation hole in some cases (See Japanese Utility Model Registration Application Publication No. S63-39703).
With regard to this filter member, a trapping diameter and an area are usually considered so that desired foreign-substance trapping performances and ventilation performances are obtained, but if a desired ventilation amount cannot be obtained with one ventilation hole for atmospheric pressure control, a plurality of ventilation holes are provided and a filter member is installed in each. Particularly, in the case of a large-sized pellicle for liquid crystal manufacture such as those having a side exceeding 500 mm, eight to several tens of ventilation holes are usually provided and a filter member is installed in each in order to ensure the ventilation amount. However, to provide a large number of ventilation holes in order to ensure the ventilation amount results in deterioration of rigidity of the pellicle frame, which is not preferable. If the rigidity is lowered, deformation can easily occur during manufacture, deflection of the frame when the pellicle film is extended becomes large, which leads to a problem that an effective exposure area inside the pellicle is reduced or accuracy of pellicle attachment is deteriorated.
As the filter member used here, a sheet-shaped one in which a porous thin film obtained by stretching and working polytetrafluoroethylene (PTFE) or the like is used as a filter film is suitable for use in the pellicle in general in terms of cost and the shape and often used. The structure is a three-layered structure as shown in FIG. 4B, in which a pressure-sensitive adhesive layer 43 to be bonded to a pellicle frame is arranged in a ring shape on one face of a filter film 41, and a rough mesh-state protective mesh 42 made of polypropylene (PP) or the like in order to protect the filter film 41 is bonded to the opposite face, and its entire thickness is approximately 0.15 to 0.3 mm. FIG. 4A is a plan view, FIG. 4B is a front view, and FIG. 4C is a bottom view of an example of a filter member. A work of applying an adhesive substance to the filter film or infiltrating the pressure-sensitive adhesive substance might be performed in order to prevent dust generation in some cases (See Japanese Patent Laid-Open No. H9-68792).
In this case, since the filter film is covered by the pressure-sensitive adhesive substance, foreign substances having adhered during manufacture of the filter member is fixed on the filter film and cannot move into the pellicle during ventilation, and dust generation from the filter member is drastically lowered. An application amount of the pressure-sensitive adhesive substance is determined considering ventilation characteristics of the filter film, but a dust-generation prevention effect and ventilation characteristic have a contradicting relationship. That is, the larger the application amount is, the better the dust-generation prevention effect, but the ventilation characteristics are deteriorated. Thus, it has been extremely difficult to ensure high ventilation capacity while a high dust-generation prevention effect is maintained.
Therefore, in order to ensure ventilation characteristics particularly in a large-sized pellicle, it is necessary to increase the number of ventilation holes for atmospheric pressure control and to accordingly increase the number of installed filter members. However, if the number of installed filter members is increased, various problems occur such as a burden of an installation work or the like.
The filter member 56 is usually installed on the outer surface of the pellicle frame 51 so as to cover the ventilation hole 52 as shown in FIG. 5B. A pellicle film 54 is attached on an upper end face of a pellicle frame 51 via a film adhesive layer 53 and a mask pressure-sensitive adhesive layer 55 is disposed at a lower end of the pellicle frame 51. FIG. 5A is a front view and FIG. 5B is a sectional view by I-I line of a prior-art pellicle.
If a large number of filter members protrude outside the pellicle frame, a risk is considerably increased that the filter member might touch a handling jig, a pellicle attaching device or an exposing device and is damaged or the pellicle frame is pushed inward for a thickness of the filter member and this causes a wrinkle in the pellicle film.
Moreover, there is a problem in manufacture. The filter member needs to be installed with tweezers or the like by well aligning the position so as not to block the ventilation hole. However, since the filter member is usually as thin as approximately 0.15 to 0.3 mm, the member should be attached with utmost care so that the film surface is not stained or damaged, and the work is extremely difficult. Therefore, if the number of members to be attached is large, the filter member attachment work has a particularly big burden on workers.